A massive densification of radio access nodes has been argued as a potential solution to meet the spectral efficiency requirements expected in future radio access networks. However, a recent study has showed that a plain network densification would significantly increase the overall energy costs. Therefore, future generations of dense radio access networks should be co-designed to be both spectral- and energy-efficient.
A new design feature that promises to fulfil these requirements is an adaptive on/off duty-cycling of radio access nodes to traffic or other relevant network statistics. Future releases of the related art 3GPP Long Term Evolution (LTE) system may adopt this feature for heterogeneous networks where a large number of small cells (i.e., cells with a small coverage area), deployed within the coverage area of a macro-cell, are switched on to offload the macro-cell when the latter is overloaded, whilst they are switched off or transit in a low-power (dormant) state otherwise.
The European project EARTH has analyzed the energy efficiency of a reference network comprising state-of-the-art macro, pico, micro, and femto/home base stations, showing that a truly energy efficient utilization of such nodes in a dense deployment requires to fully de-activate the baseband processing and the radio hardware. Hence, rather than completely switching off a radio access node, an alternative solution is to introduce discontinuous transmission/reception (DTX/DRX) at the network side, i.e., to put the radio access nodes in an idle (low-energy consumption) mode with limited transmission/reception capabilities whenever suitable. The EARTH project has further observed that in average a cell is actively transmitting user data about 5% of the subframes. This number may be significantly lower in ultra dense networks, e.g., 1% or less. An idle mode DTX ratio of 1% implies that for a transmission time in the order of tens/hundreds of milliseconds (e.g., 100 ms) the idle time would be in the order of tens of seconds (e.g., 10 s), depending on traffic statistics, user migration etc.
One problem with introducing long sleeping cycles followed by very short activation time of radio access nodes is that the traditional cell discovery procedures become inefficient. For instance, the cell search procedure in the related art LTE system is designed assuming that base stations (eNodeB) are constantly active and consists of a series of synchronization steps upon which a mobile station (User Equipment, UE) acquires time and frequency synchronization and other crucial system parameters that are necessary to demodulate other downlink signals.
The LTE cell-search exploits two specially designed signals: the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). The particular design of these signals allows a UE to acquire the correct time-frequency synchronization (including subframe boundaries), as well as the physical identity of the cell, the cyclic prefix length, and whether the cell operates in Time Division Duplex (TDD) or Frequency Division Duplex (FDD).
The PSS/SSS synchronization signals are transmitted twice per 10 ms radio frame for both TDD and FDD cases, although with slightly different time structure. A cyclic prefix is added to both PSS and SSS and the length of the synchronization signals are blindly detected. While the PSS is the same in both transmissions within a radio frame, the SSS can change within a radio frame in a specific manner to enable the UE to detect the position of the 10 ms radio frame boundary. In the frequency domain, the PSS/SSS are mapped to subcarriers within the six central resource blocks, thus being invariant to the system bandwidth and enabling the UE to synchronize to the network without prior knowledge of the exact system bandwidth. The PSS and SSS signals consist of a length-62 symbol Zadoff-Chu sequence mapped to the central 62 subcarriers around the d.c. subcarrier (which is left unused). The two sequences used for PSS and SSS in a given cell are specially designed to reveal the physical layer cell identity to the mobile station. In particular, in LTE there are 504 physical cell identities grouped into 168 groups of three identities (typically assigned to three cells under the control of the same eNodeB). Three PSS sequences are used to indicate one cell identity within a group, while the 168 SSS sequences are used to identify a group.
The PSS and SSS are always transmitted from the same antenna port in any given subframe, while between different subframes they may be transmitted from different antenna ports in case the eNodeB uses multiple antennas. The requirements for cell detection combine a minimum SINR condition with a maximum allowed detection time, e.g., (−6 dB, 800 ms) for intra-frequency with a measurement period of 200 ms.
Dynamic on/off switching of network cells has been considered as potential technique for interference mitigation in dense deployments of small cells in the related art LTE system. One approach is to enable base stations to transit into an idle mode with limited transmission/reception capabilities, hence with low-energy consumption, followed by a very short active time for transmission/reception.
The current cell detection supporting the network access procedure in the related art LTE system is designed for a sparsely deployed system of macro-cells always active. The procedure comprises a first detection step based on synchronization signals (PSS/SSS) and a second step based on Common Reference Signals (CRS) used to verify the cell ID and perform initial Reference Signal Received Power (RSRP) measurements. The requirements for cell detection, listed in Table 1, combine a minimum Signal-to-Interference and Noise Ratio (SINR) condition with a maximum allowed detection time, e.g., (−6 dB, 800 ms) for intra-frequency with a measurement period of 200 ms. The measurement sampling is implementation specific, but typical values range in 1-2 ms sample/snapshot per 40 ms or per DRX cycle.
TABLE 1LTE cell detection requirements in RRC_CONNECT stateE-UTRAN intra-frequencyE-UTRAN inter-frequencyReceivedCRS: Es/IoT ≥ −6 dBCRS: Es/IoT ≥ −4 dBsignalPSS/SSS: Es/IoT ≥ −6 dBPSS/SSS: Es/IoT ≥ −4 dBqualityMeasuredUp to 7 cellsUp to 3 inter-frequencies andcellsup to 4 cells per frequencyTimeDetection time: 800 msDetection time: requirements(3.84 s, 7.68 s) · NfreqMeasurement period:Measurement period:200 ms480 ms · Nfreq
In a dense synchronized network, the number of small cells detected by a UE with a legacy LTE cell-search is limited by the interference: a) the PSS/SSS from different cells occupy the same time-frequency resources, hence the PSS/SSS of one cell in a dense deployment is likely to collide and interfere with other PSS/SSS (due to the small radius of the cells). As only 3 PSS sequences are used in LTE, the coherent detection of PSS/SSS is degraded by these collisions; b) the maximum time difference of different PSS/SSS signals is seldom larger than one cyclic prefix; c) the time period available for combining multiple samples is reduced due to the limited coverage of small cells. Therefore, the legacy LTE cell search is not sufficiently robust in a dense network deployment. Furthermore, depending on the SINR at the receiver, the detection time can take several hundreds of milliseconds (ms). Hence, when base station operate with a long sleeping duty-cycle followed by a short activation time of a few hundreds of ms, the legacy LTE cell-search would not have sufficient time to correctly detect the cell signals and to perform measurements.
A straightforward way to improve the performance of the legacy cell-search is to relax SINR requirements and use legacy synchronization signals for RRC_CONNECETD UEs with reduced number of samples. This, however, requires longer cell-search and only circumvents the problem without actually solving it.
Another approach is to consider network-assisted synchronization, i.e., to assume that the UE knows which particular subframes are used for transmission of detection signals. This, however, requires the UE to be aware of the resources configured for detection signals.
Another method is to reduce the density of the synchronization and reference signals, and hence reduce the inter-cell interference on PSS/SSS when a large number of cells is deployed in a small geographical area. For instance, a network-assisted mechanism can be used to time multiplex the transmission of PSS/SSS/CRS and MIB/system information (SI) from different cells in N milliseconds (ms) bursts every M ms with L ms offset compared to a serving cell, with N, M, L configured by the active serving cell. A drawback of this method is that implies a serving (macro) cell supposedly active at all times.
A different approach is to design new discovery signals for a mobile node in RRC_CONNECTED state. One method is to use synchronized transmission of discovery signals among cells in order to reduce the discovery time and enhance UE energy efficiency. To this end, two possible reference signals have been proposed for the LTE system: channel-state information reference signals (CSI-RS); and Positioning Reference Signals (PRS). With the former, each small cell within a cluster would transmit a CSI-RS of a different configuration pattern while muting the CSI-RS resources for all other configuration patterns, thereby enabling fully orthogonal discovery RS within a cluster. In the latter case, a small cell would transmit the PRS with sub-carrier shifts of reuse factor 6 according to the physical cell ID (PCI). In either case, the mobile station needs to know a priori the carrier, bandwidth, and time where these signals are transmitted for each cell.
Other methods to improve the energy efficiency of the cell discovery in case of dynamic on/off switching of base stations is to use uplink signals transmitted by the UE to trigger the downlink transmission of discovery signals from neighbouring base stations in idle state. This, however, requires new uplink signals to be designed, as well as a new handover mechanism between access nodes based on signal strength measurements carried out by and exchanged across multiple the access nodes, rather than multiple reports from a mobile station to a serving cell. Furthermore, when the UE is in RRC_CONNECT state, i.e., connected to a serving cell, new criteria should be identified for self-triggering the transmission of such uplink discovery signals.
As been discussed above, with the introduction of the idle mode capabilities, some or all the signals that are typically transmitted to aid the mobile station to detect a network node, synchronize to, and access the network may either be absent or transmitted only sporadically. Detecting the presence of a dormant cell may therefore require longer time, thereby draining the battery of the mobile station. On the other hand, the prior art procedures are not sufficiently fast to enable a mobile station to quickly connect to a dormant cell when it switches to an active mode for a short time period.
Moreover, in a radio access network with a dense deployment of radio access nodes, conventional cell detection procedures based on synchronization signals become ineffective due to the inter-cell interference. When radio access nodes are further enabled to transit into an idle mode with limited transmission/reception capabilities and are active only for a short time (e.g., 1% active duty cycling), an additional issue is to assure fast cell detection prior a network node is turned off again.
A further related issue is the energy efficiency of the cell-search procedure at the mobile node, as a mobile station failing to detect a cell in its active time may need to extend the cell search until the cell reactivates.